Field of the Invention
The present invention relates generally to a ceramic strain sensor, and more particularly to such a ceramic strain sensor having a ceramic substrate which is deformed in response to a stress exerted thereon, and an array of resistors whose resistance value varies depending upon a magnitude of deformation of the ceramic substrate, whereby the magnitude of deformation or strain and the corresponding stress are detected or determined.
Discussion of the Prior Art
There is known a strain sensor or detector of a type wherein resistors connected in a bridge circuit are formed on a deformable substrate such as a diaphragm, which is deformed by a stress such as a pressure applied thereto. Deformation of the diaphragm results in a certain degree of strain of the resistors, thereby causing a loss of balance of the bridge of the resistors in relation to the magnitude of deformation of the substrate and the stress exerted thereto. According to this principle, the strain sensor is capable of measuring the magnitude of a strain or a stress which causes the strain. As a strain sensor of this type, there has been recently proposed a ceramic strain sensor which utilizes properties of a ceramic material, such as high heat resistance, which permits the sensor to operate at an elevated temperature.
An example of a pressure sensing device using such a ceramic strain sensor is disclosed in U.S. Pat. No. 4,311,980 (corresponding to Publication No. 62-12458 of examined Japanese Patent Application). In this pressure sensing device, the strain sensor includes a ceramic substrate which is deformable due to a pressure applied thereto, and a plurality of thick-film resistors formed on the ceramic substrate by a screen printing technique. The strain sensor incorporates an electric circuit adapted to detect a change in the resistance values of the resistors, which varies as a function of the deformation or strain of the ceramic substrate.
In the pressure sensing device including the known strain sensor of the type indicated above, the resistors formed on the ceramic substrate consist of a base formed of a dielectric material such as borosilicate, aluminosilicate, lead borosilicate and lead silicate, and at least one electrically conductive component as a principal conductive material or materials, which is selected from the group consisting of RuO.sub.2, IrO.sub.2, TlO.sub.2, Bi.sub.2 Ru.sub.2 O.sub.7, Au, Pt, Pd, and alloys containing at least one of these compounds. Since the resistors of the above composition are formed by firing at a temperature in the neighborhood of 800.degree. C., the resistors are damaged or deteriorated and become incapable of normally functioning, when exposed to several hundreds of degrees centigrade. For this reason, the upper limit of the temperature at which the sensor can normally function is limited to about 200.degree. C.
Thus, the pressure sensing device disclosed in the above-identified publication is not satisfactory in its heat resistance, and therefore suffers when the device is used to detect a pressure at a pressure source in high temperature environments, for example, a pressure in an engine of an automotive vehicle. Namely, a fluid having the pressure to be detected should be directed from the pressure source to a suitable detecting location at which the temperature of the fluid is sufficiently lowered. In this case, a passage for directing the fluid to the detecting location tends to be plugged by combustion products produced by the engine, such as carbons, and the measurement of the pressure becomes difficult.
On the other hand, the ceramic substrate and the thick-film resistors formed on the substrate cannot be co-fired, in the strain sensor discussed above. Accordingly, the process of producing the strain sensor requires that an appropriate unfired composition for the thick-film resistors be applied by printing to the fired ceramic substrate, and the unfired resistor material be subsequently fired. These process steps are time-consuming, and the strain sensor arrangement produced by the process is practically limited in construction.